An active matrix substrate used in a liquid crystal display device, or the like, includes a switching element such as a thin film transistor (hereinafter, a “TFT”) in each pixel. It has been proposed to use a TFT whose active layer is an oxide semiconductor layer (hereinafter, referred to as an “oxide semiconductor TFT”) as the switching element.
An oxide semiconductor TFT includes a protection layer (passivation layer) formed on an oxide semiconductor layer by a CVD method or a sputtering method using a plasma, for example, in order to suppress deterioration of TFT characteristics over time. When forming the protection layer, however, the surface of the oxide semiconductor layer may possibly be damaged. Specifically, oxygen deficiency may occur in the oxide semiconductor layer or hydrogen may diffuse from the protection layer, thereby lowering the resistance (conductorization) of the surface of the oxide semiconductor layer. When the resistance of the oxide semiconductor layer lowers, the threshold voltage greatly shifts toward the negative side (depletion characteristic), and desired TFT characteristics may be not realized.
In view of this, it has been proposed to perform an oxidation treatment such as an N2O plasma treatment on the oxide semiconductor layer immediately before the formation of the protection layer. For example, by irradiating the oxide semiconductor surface with an N2O plasma to oxidize the surface of the oxide semiconductor layer, it is possible to reduce the damage to be inflicted upon the oxide semiconductor layer during the formation of the protection layer.
However, if the surface of the source and drain electrodes of the oxide semiconductor TFT is exposed when the N2O plasma treatment is performed, the exposed electrode surface may possibly be exposed to the N2O plasma and oxidized. For example, Patent Document 1 states that when copper (Cu) or a Cu alloy is used as the electrode material, an oxide film may be formed on the electrode surface through the N2O plasma treatment.